1. Field of the Invention
This invention relates to cardioplegia catheters having elongated inflatable cuffs for improved cardioplegia distribution.
2. Description of Related Art
Since the early days of cardiac surgery, it has been recognized that in order to provide the optimum surgical conditions when operating on the heart, it is necessary to interrupt the normal operation of the heart. For obvious reasons, an arrested, flaccid heart is preferred during cardiac surgical procedures over a beating heart with blood flowing through it. Thus, in order to be able to efficiently perform cardiac surgery, it is often necessary to use cardiopulmonary-bypass techniques and to isolate the heart from its life-giving blood supply.
The heart is divided into a left and a right half. The right side of the heart is smaller and pumps deoxygenated blood returning from the body into the lungs. The left side of the heart performs the bulk of the work, and pumps oxygenated blood from the lungs to the rest of the body. Each side of the heart has an upper chamber, called an atrium, and a lower chamber, called a ventricle. The atrial chambers accumulate blood and supply it to the ventricular chambers, which actually do the pumping work.
The ventricular chambers operate by expanding and drawing blood in from the atriums, then contracting and forcing the blood out. Inlet and outlet check valves keep the blood moving in the right direction. The right atrium and right ventricle are connected through the tricuspid valve. The left atrium and left ventricle are connected through the mitral valve. The right ventricle pumps blood out through the pulmonary valve, and the left ventricle through the aortic valve.
Deoxygenated blood returns from the body to the heart by means of the vena cava into the right atrium. During the heart's diastole (expansion), the blood is drawn through the tricuspid valve into the right ventricle. During the heart's systole (contraction), blood is forced from the right ventricle, into the pulmonary artery and on into the lungs to be oxygenated. Oxygenated blood returns from the lungs by means of the pulmonary vein, and enters the left atrium. During the heart's systole, the blood is forced out of the left ventricle, through the aortic valve and into the aorta, from whence it branches off to serve all areas of the body including the heart itself.
The blood supply which serves the heart muscle with oxygenated blood originates from two openings, called coronary ostia, in the aorta near the aortic valve. From the coronary ostia, the blood flows through the coronary arteries, and branches off into a myriad of tiny capillaries to provide oxygenated blood to all areas of the heart muscle. Approximately 80% of the blood entering the coronary arteries drains through veins into the coronary sinus vein, which in turn drains into the right atrium. The remaining blood drains by alternative passages into the heart's chambers, primarily the right atrium.
During cardiac surgery, the heart is isolated from the circulatory system and the patient is connected to a heart-lung machine which oxygenates and pumps blood. A venous catheter is inserted into the right atrium and drains blood returning from the body into the heart lung machine. An arterial cannula is inserted into the aorta, so that oxygenated blood from the heart-lung machine can be pumped back into the body. After these catheters are in place, the aorta is cross clamped between the arterial cannula and the heart, to prevent blood from flowing backwards into the heart. Cross clamping involves pinching the aorta closed with a clamp having elongate jaws which extend the full width of the aorta. The foregoing procedure provides blood to all areas of the body except the heart, because the aortic clamping prevents oxygenated blood from the heart-lung machine from entering the coronary arteries. Accordingly, some method must be provided for preventing degradation of the heart tissue during the surgery.
One of the early methods utilized to protect the heart muscle during surgery was normothermic (body temperature) perfusion of the empty beating heart. This method was utilized in an effort to maintain the heart, as much as possible, in normal conditions during surgery. Although this procedure eliminated the problem of blood flow, dissection and suturing were still difficult to perform because of the firmness and the beating of the heart. Additionally, it was found that a significant amount of damage still occurred to the heart muscle when this procedure was utilized.
A second method developed to protect the myocardium was intermittent cardiac ischemia (stoppage of blood flow to the heart muscle) with moderate hypothermia. This method requires that the entire body be cooled to a temperature from 28.degree. C. to 32.degree. C., thus slowing all bodily functions, including those of the heart. The heart is electrically stimulated to induce fibrillation (mild fluttering) before aortic cross-clamping to stop the beating. The surgeon can then operate for approximately 15-25 minutes, after which time the heartbeat is necessarily resumed for 3-5 minutes. This procedure proved to be an inefficient method for performing operations and had many attendant dangers, including fibrillation of the heart.
A third method which has been utilized is profound hypothermic cardiac ischemia. This method requires that the temperature of the heart be lowered to about 22.degree. C. by the infusion of a cooled perfusate and/or by filling the pericardium (chest cavity containing the heart) with cold saline solution. One of the major disadvantages of this technique is that the heart continues to fibrillate, exhausting the heart-stored energy. As a result, the heart becomes acidotic, which over time causes irreversible muscle damage.
Currently, the most common method to preserve the myocardium during surgery is the infusion of a cold cardioplegic fluid to both cool the heart and stop it from beating. After the initial infusion, the heart is reperfused approximately every 30 minutes to maintain the cool, dormant state of the heart. Alternatively, a continuous flow of cardioplegic solution may be provided.
The use of cardioplegia, which literally means "heart stop," to protect the myocardium has proven the most advantageous method of those used to date. Cardioplegia solution may be administered in an antegrade manner (through arteries in the normal direction of blood flow), in a retrograde manner (through veins opposite the normal blood flow direction), or in a combination of retrograde and antegrade administration. Cardioplegic solutions, typically containing potassium, magnesium, procaine or a hypocalcemic solution, stop the heart by interfering with the heart's capacity to conduct the natural electric signals which tell it to beat.
In normal antegrade cardioplegia, a single needle is inserted into the aorta beneath the cross-clamp, and the cardioplegic solution is administered therethrough. The cardioplegic solution flows through the coronary arteries in the normal blood flow direction. Care must be taken to avoid mechanical injury to the coronary ostia which could produce the serious complications of coronary ostial stenosis (i.e., constricting of the coronary ostia). Ostial stenosis requires reparative surgery and can be quite hazardous due to obstruction of the coronary arteries. Moreover, it is a nuisance to have perfusion catheters present within the limited operative field during aortic valve replacement.
Retrograde cardioplegia is conventionally administered by inserting a balloon catheter into the coronary sinus, inflating the balloon, and perfusing the cardioplegic solution backwards through the coronary veins. Typically, catheters for retrograde coronary sinus perfusion (RCSP) may contain either a manually inflating, or auto-inflating balloon or cuff. A manually inflating cuff is filled through an inflation lumen, either attached to the outer surface of the cannula body, or integral to the cannula body. Typically, a syringe supplies the inflation fluid.
An auto-inflating cuff is filled by a flow of cardioplegic solution. Several methods are used to fill the auto-inflating cuff with cardioplegic solution, but all rely on the principle of a flow restriction downstream of the cuff to provide a back pressure for filling the cuff. In one design, the infusion lumen is plugged; the cuff has one or more inlets from the infusion lumen upstream of the plug and one or more outlets to the infusion lumen downstream of the plug. The combined surface area of the cuff outlets is less than the combined surface area of the cuff inlets, thereby providing sufficient back pressure to inflate the cuff. In this design, all of the cardioplegic solution flows through the interior of the cuff.
Because the coronary sinus is susceptible to damage from high pressures, some form of pressure monitoring should be employed during RCSP. Typically, a separate pressure lumen, either integral with the catheter body, or external to the catheter body, is provided. One or more openings are typically provided at or near the distal end of the cannula, so that the pressure lumen is in communication with the coronary sinus for accurate pressure monitoring. A separate opening, in communication with the infusion lumen, is sometimes provided as a security feature in case the primary openings to the coronary sinus become occluded.
RCSP offers several advantages over antegrade cardioplegia delivery. It avoids arterial ostial stenosis, there is no need to interrupt surgery for re-infusion, it allows prolonged cardioplegia delivery due to the low flow rates, and provides good uniformity of cardioplegia distribution throughout the heart. Menasche, P. and Piwnica, A. H., "Retrograde Coronary Sinus Perfusion," Roberts Textbook of Myocardial Protection in Thoracic Surgery. Chap. 15, pp. 251-262 (1987).
One of the drawbacks of RCSP is a lower flow to the veins serving the right ventricle and atrium. While the coronary sinus drains a majority of the heart's blood supply, the remainder goes through alternative blood vessels, termed the arteriosinusoidal and thebesian vessels, directly into the cardiac chambers. Typically, these alternative systems serve the right ventricle and atrium. Thus, these areas of the heart will not receive as much cardioplegic solution during retrograde administration. An additional mechanical limitation arises from several small veins, from the right side of the heart, emptying into the coronary sinus as close as 0.5 cm. to the coronary sinus ostium. Id.
Of primary concern is the posterior left coronary vein, a relatively large vein which drains into the coronary sinus adjacent its ostium. To ensure adequate retention of the inflated cuff within the coronary sinus, the cuff must generally be placed into the coronary sinus at a point beyond where the left coronary vein enters the coronary sinus. The left coronary vein is thus left open to the right atrium. The arterial and venous systems serving the heart rapidly branch into small capillaries through which there are many interconnections. There is concern that the left coronary vein may act as a shunt which allows cardioplegia fluid entering the coronary sinus at a pressure of 40 mm Hg to seek the atmospheric pressure of the right atrium through the left coronary veins, thus bypassing more distal regions of the coronary venous system.
As previously described, dislodgement of the catheter from its position within the coronary sinus is a further concern. Typically, as the cuff is inflated to a higher pressure to obtain a higher retentive force, it becomes more rigid and loses elasticity in the axial direction. Axial forces accidentally applied to the catheter during the operation are not absorbed by the cuff but are transmitted directly to the wall of the cuff in contact with the coronary sinus. This can result in the cuff becoming dislodged from its position within the coronary sinus.
The catheter of the present invention employs an inflatable cuff which overcomes these problems by purposely occluding the left coronary vein during the perfusion procedure to prevent cardioplegia fluid from "leaking" out of the coronary sinus through the left coronary vein. Additionally the design of the cuff allows it to absorb axial forces applied to the catheter to inhibit dislodgement of the catheter from the coronary sinus.